1. Field of the Invention
The present invention relates to a nickel-hydrogen stacked battery pack. In particular, the present invention relates to a nickel-hydrogen stacked battery pack which has a high capacity and can be produced at low costs.
2. Description of the Related Art
The voltage of a single nickel-hydrogen cell reaches only 1.2 V at which water is stabilized in the case of a metal hydride secondary cell. Therefore, a plurality of cells are connected in series and used in a high voltage state practically, and are often supplied in the form of a battery pack.
A conventional nickel-hydrogen battery pack comprises a plurality of cylindrical cells connected in series. For example, as shown in FIGS. 1A and 1B, three cylindrical cells 30 are connected in series to generate a nominal voltage of 3.6 V. In this case, the cylindrical cells as element cells can be mass produced at low costs, but the battery pack consisting of the cylindrical cells has many dead spaces between the cylindrical cells 30 as seen from FIG. 1A. Therefore, it is difficult to increase the capacity per unit volume of such the battery pack.
To remove such the drawback of the battery pack consisting of cylindrical cells, it is proposed to produce hexahedral element cells having a rectangular cross section and connect a plurality of hexahedral element cells in series. For example, as shown in FIGS. 2A and 2B, three hexahedral cells 40 are connected in series to generate a nominal voltage of 3.6 V.
The battery pack consisting of the hexahedral cells has no dead space, since each element cell has flat peripheral surfaces as shown in FIG. 2A.
The hexahedral element cells have a structure that the opening of a hexahedral case containing power-generating elements such as positive and negative electrode, separators and an electrolytic solution is sealed with a sealing plate having a reversible vent. Therefore, the volume of each element cell becomes large in relation to the filled volume of the power-generating elements, and thus the increase of capacity is limited. Furthermore, the hexahedral case and the sealing plate having the reversible vent must be supplied for assembling each element cell since the opening of each cell case is completely sealed with the sealing plate. In addition, the sealing of the opening requires some costs, and therefore the production costs increase.